Conflict in full swing: Forest bats avoid large areas around fast-moving wind turbines

More and more wind turbines are being installed worldwide in order to meet the need for an increase in the proportion of renewable energy sources in response to the goals of national climate strategies. In Germany, around 30,000 onshore wind turbines are currently in operation, and as suitable locations become increasingly scarce, the search for sites now also extends to additional, potentially less suitable locations. These include forests as potential sites -- and with these diverse and sensitive habitats. Many European bat species, such as the greater mouse-eared bat (Myotis myotis), live and forage in forests and are therefore potentially affected by the expansion of wind energy in or near forests. A new scientific investigation by a team led by Christian Voigt from the Leibniz-IZW and Nina Farwig from the Philipps-Universität Marburg shows that this not only poses the direct risk of colliding with the wind turbine rotors, but also has indirect negative effects on these species. The scientists found that the forest specialists among the bats avoid wind turbines over a distance of several hundred metres once the turbines are in operation and the wind speeds are relatively high.

"We investigated the activity of different bat species in different wind conditions and during the operation of wind turbines in forests in the German federal state of Hessen," says Julia Ellerbrok, a former doctoral student in the project and now a postdoctoral research fellow in the Department of Biology at the Philipps-Universität Marburg. "We found that the activity of bats, which usually forage in narrow, structurally dense vegetation of forests, decreases by 77 % on average within a radius of 80 to 450 metres around the wind turbines with increasing wind speed when the turbines are in operation. In contrast, bat activity was unaffected by wind speed when the turbines were switched off." The team therefore concludes that factors directly related to the operation of the turbines at relatively high wind speeds must be responsible for such avoidance behaviour.

"The rotor movements of wind turbines not only generate so-called wake turbulences, but also substantial noise. Both factors can affect bats over several hundred metres," says Christian Voigt, head of the Department of Evolutionary Ecology at the Leibniz-IZW. "Forest bats that hunt under the canopy presumably do not come into contact with the wake vortices. Rather, they could be affected by the noise emissions of the turbines, even if the frequency range of the noise is far below those of the echolocation calls. If bats actively avoid noise emissions from wind turbines, they lose valuable habitat on a large scale."

Wind turbines in forests pose several problems for bats, the scientists sum up: Not only do forest bats lose valuable habitats -- both through clearing during the construction of the wind turbines and by avoiding the wind turbines once they are in operation. Bats that hunt above the treetops can also be potentially killed by the rotating blades. In order to minimise the potential long-term ecological impact of wind turbines on bat populations in forested areas, wind turbines should only be erected in structurally poor forest plantations where only few bats live. Future research should focus on investigating the effects of noise emissions from wind turbines on bats in more detail, the team concludes in the paper.